In recent years, a compact disk (hereinafter, referred to as a CD) has been in common use, and accordingly a writing-once type optical disk (hereinafter, referred to as a CD-R) which meets the CD standard has been developed and used. The CD-R uses a disk-shaped transparent resin substrate having a spiral groove transferred by means of a stamper upon the time of forming. After the substrate is coated with an organic dye (or pigment) solution by a spin coater, a light absorption layer (or a recording layer) is formed by drying the solution, a reflective layer is formed on the light absorption layer by for example sputtering gold, and a protective layer is formed by coating the reflection layer with an ultraviolet curable resin by a spin coater or the like followed by curing the resin. The CD-R is composed of the substrate, the light absorption layer, the reflection layer and the protective layer. The optical recording medium, in which the light absorption layer is thus formed by coating the substrate with the organic dye solution is also referred to as a coating type optical recording medium.
Information is recorded on the CD-R in such a manner that pits corresponding to the information is formed in the optical recording medium in accordance with any combination of change in properties of the organic dye caused by irradiation of the light absorption layer with a laser beam having a wavelength of about 780 nm, decomposition and change in optical constants of the organic dye caused by such irradiation and deformation of the substrate due to heat generation at the time of the decomposition or the change in properties. Moreover, the recorded information is reproduced by detecting a change in an amount of a light reflected from the pits. Such manners of recording/reproducing information on/from the optical recording medium is well-known.
In the case where information is recorded on or reproduced from the CD-R by using a CD-R writer for recording/reproducing, or in the case where information is reproduced by using a CD player or a CD-ROM drive, the optical recording medium needs to a stable servo characteristic which allows a pick-up member to move promptly to a predetermined position along a groove of the substrate.
In order to stabilize the servo characteristic, it is important to improve particularly a tracking error (TE) signal and a radial contrast (RC) signal among various characteristics of the CD-R. Moreover, it is necessary to produce a high-quality reproduction signal at the time of the reproduction, and it is particularly important to obtain a stable output with less fluctuation. Factors which greatly influence these characteristics are as follows: as for the tracking error signal, a difference in level between a light absorption layer formed on a groove section formed on the substrate having the groove section and a light absorption layer formed on the inter-groove section (a portion of the substrate between the adjacent groove sections); as for the radial contrast, a width of the groove section on the substrate; and as for the fluctuation of the output signal, fluctuation in a thickness of the light absorption layer (particularly, fluctuation along a radial direction of the substrate).
For the coating type CD-R, in order to ensure these properties, the following is performed. In order to form the light absorption layer on the substrate, a substrate of which groove is comparatively deep is used because when the substrate is coated with the organic dye solution, the solution is likely to preferentially fill the groove of the substrate. Since the groove depth is likely to be shallower along a direction from an inner periphery to an outer periphery of the substrate at the time of the substrate production, such a substrate having the deep groove is produced so as to suppress such inclination (or slope) of the groove depth by adjusting substrate production conditions (for example, a temperature of a melted resin, temperature distribution of a metal mold and the like).
In addition, since a thickness profile of the light absorption layer is inclined along a direction from an inner periphery to an outer periphery of the disk due to evaporation of the organic dye solution at the time of fabrication process of the light absorption layer of the optical recording medium by spin-coater, the light absorption layer is formed so as to suppress the inclination of the thickness profile while precisely adjusting the conditions of the application of the organic dye solution (for example, adjustment of viscosity of the organic dye solution, control of rotation of a spin coater and the environmental conditions at the time of production), the conditions of drying the solution and the like, whereby ensuring a necessary quality level.
In addition, in order to achieve a higher density than that of CD, a digital video disk (hereinafter, referred to as DVD) for a laser beam having a wavelength of about 650 nm has been developed and commercially sold recently. Similarly to CD, this DVD is produced in the following manner: an optical recording layer, which has pits with a track pitch (TP) of 0.74 .mu.m (about half of TP of CD (1.60 .mu.m)) and with a shortest pit length of 0.40 .mu.m, is provided on a resin substrate having a thickness of 0.6 mm by a stamper of a molding machine; a reflective film is provided on the optical recording layer; and another resin substrate having a thickness of 0.6 mm is laminated thereon by using an adhesive such as an ultraviolet curable resin in order to ensure strength suitable for the disk. As to such DVD, it is also desired to develop an optical recording medium (optical disk) which is able to write once and realizes the high density similarly to the write-once type CD-R, namely DVD-R.
For such a write-once type high density optical recording medium, pits corresponding to the information has to be stably formed and good quality signals have to be obtained from the pits at the time of reproduction by using a substrate having a narrower groove with a small track pitch less than a half of the track pitch of the CD-R. For this, when the track pitch and groove width of the substrate become smaller, the inclination profile of the groove depth along the radial direction becomes more larger in the substrate having the deep groove used in the coating type optical recording medium. Moreover, since the groove width becomes narrower, the inclination of the thickness profile of the light absorption layer becomes more larger. Due to the above, it is difficult to suitably ensure the aforementioned properties only by adjusting the substrate production conditions, the conditions of the application of the organic dye solution, the conditions of drying the solution and the like.